The present invention relates generally to milling a casing exit for a lateral borehole, and more particularly to systems and methods of protecting the lowside of the casing from wear while milling a casing exit for a lateral borehole.
Hydrocarbons can be produced through relatively complex wellbores traversing a subterranean formation. Some wellbores can include multilateral wellbores and/or sidetrack wellbores. Multilateral wellbores include one or more lateral wellbores extending from a parent (or main) wellbore. A sidetrack wellbore is a wellbore that is diverted from a first general direction to a second general direction. A sidetrack wellbore can include a main wellbore in a first general direction and a secondary wellbore diverted from the main wellbore in a second general direction. A multilateral wellbore can include one or more windows or casing exits to allow corresponding lateral wellbores to be formed. A sidetrack wellbore can also include a window or casing exit to allow the wellbore to be diverted to the second general direction.
The casing exit for either multilateral or sidetrack wellbores can be formed by positioning a casing joint and a whipstock in a casing string at a desired location in the main wellbore. The whipstock is used to deflect one or more mills laterally (or in an alternative orientation) relative to the casing string. The deflected mill(s) penetrates part of the casing joint to form the casing exit in the casing string. Drill bits can be subsequently inserted through the casing exit in order to cut the lateral or secondary wellbore.
While milling the casing exit, however, and during drilling of the subsequent lateral wellbore, significant wear can result on the lowside of the parent wellbore casing at or near the tip of the whipstock. The wear on the lowside of the wellbore is partly generated by the mills as a reactive force while cutting the exit in the casing or while trying to exit into the formation. Considerable wear is also generated by the drill pipe as it lays and rotates on the lowside of the parent wellbore at or near the tip of the whipstock.
In applications where the casing joint is made of softer casing materials, such as aluminum, the resulting wear can be significant. However, in instances where it is difficult for the casing exit to be milled, or there is a significant amount of time spent rotating the drill pipe at or near the tip of the whipstock, there can be significant wear even in steel casing (e.g., low alloy steel or 13Cr). This wear oftentimes results in the formation of a ledge on the inner surface of the casing which can cause problems with other bottom hole assemblies (BHAs) transversing the whipstock and entering the lateral borehole. The damaging wear can also create problems when trying to recover the whipstock, or it could create problems for subsequent operations below the milled casing exit after the whipstock has been recovered.
Previous attempts to prevent wear on the lowside of the wellbore have focused on reducing friction with the introduction of drilling fluids or drill pipe centralizers. The success of friction reducers in drilling fluids, however, can be costly and may be environmentally prohibited depending on geographic location. Moreover, the use of centralizers can vastly increase operational time as the centralizers must be added to each stand, thereby greatly increasing trip-in time.